1. Field of the Invention
The present invention relates to the structure of pixels in an active matrix display device (in particular, a crystal liquid display device), and more particularly to the structure of an auxiliary capacitor and a black matrix (BM) connected in parallel with a pixel electrode. Also, the present invention relates to the structure of pixels in a flat panel display device that widely requires a black matrix.
2. Description of the Related Art
There has been known a display device having an active matrix circuit. This display device is structured such that a plurality of source wirings for transmitting image data, a plurality of gate wirings which are disposed to intersect with the plurality of source wirings for transmitting a switching signal, and a plurality of pixels disposed at their intersections, in which transistors (in particular, thin-film transistors) are normally used as switching elements.
Each of the pixels includes not only a transistor for switching but also a pixel electrode, and is structured such that a gate electrode of the transistor is connected to a gate wiring, a source thereof is connected to a source wiring, and a drain thereof is connected to the pixel electrode. It should be noted that the source and the drain are not regularly distinct from the viewpoint of the operation of the transistor, and alternate fluctuate depending on a signal from the viewpoint of the definition of the normal electric circuit. However, in the following description, of the impurity regions provided in the transistor, an impurity region which is merely connected to the source wiring is called "source", and another impurity region which is connected to the pixel electrode is called "drain".
Each pixel includes at least one transistor, and in particular, a pixel in which two or more transistors are connected in series is effective because a leak current can be reduced even in a condition where the transistor is in a non-selected state. Even in the above case, an impurity region which is not connected to any one of the source wiring and the pixel electrode is not particularly defined with the application of the above-mentioned definition.
The pixel electrode is disposed such that a capacitor is formed between the pixel electrode and an electrode that faces with the pixel electrode through liquid crystal. The above transistor functions as a switching element that takes out charges from the capacitor or puts the charges into the capacitor. However, in an actual operation, a value of the capacitor formed by only the pixel electrode portion is too small so that a sufficient time and necessary charges cannot be held. For this reason, an auxiliary capacitor must be additionally provided.
Up to now, this auxiliary capacitor (also called retention capacitor) was formed between an opaque electrically conductive material such as metal which is additionally provided and the pixel electrode or the semiconductor layer. Normally, a gate wiring on a succeeding line was used as an opposite electrode. However, in the case where the pixel area is large, even a capacitor formed by using the gate wiring is satisfactory, but as the pixel area becomes small, the capacitor formed by using only the gate wiring is not satisfactory, with the result that the gate wiring was required to be widened larger than it is needed in order to ensure the area of the electrode of the auxiliary capacitor. In this structure, since a portion that blocks a light exists within the pixel, the aperture ratio is lowered.